If an internal combustion engine is operated for a longer time duration in the full-load range and then is braked into the idle range, the problem is presented that the engine overheats when the idle rpm, which is usually adjusted, is maintained unchanged. An oil pressure which is too low in order to ensure adequate lubrication of the engine occurs especially because of the high oil temperatures which are then present.
To prevent the above and similar unwanted situations, U.S. Pat. No. 4,345,557 discloses that the idle speed adjustment of an internal combustion engine is carried out in the context of an idle rpm control and the desired value of this control is inputted in dependence upon the coolant temperature of the engine in such a manner that the rpm is increased in the so-called hot idle state above a predetermined engine or coolant temperature. The cooling action is amplified in this way.
U.S. Pat. No. 5,002,026 discloses the application of the oil temperature for adjusting the idle in the normal operating range. The oil temperature is determined from other variables in order to avoid the expense of providing an oil-temperature sensor. For this purpose, the time span is determined during which the coolant temperature is equal to or greater than a temperature threshold. A measure or quantity for the oil temperature is determined from a pregiven relationship between this time span and the oil temperature and the idle rpm is correspondingly adjusted. However, measures are not disclosed which, in combination with the so-called hot idle, determine the threatening drop of the oil pressure.